The broad objective of this proposal is to maintain the epidemiological framework of the Pittsburgh component of the Multicenter AIDS Cohort Study (MACS) in order to further delineate both the natural history and pathogenesis of HIV infection in homosexual men. The Pittsburgh MACS was originally funded in 1983 and charged with the broad mission of documenting the natural history of AIDS. Currently, the Pittsburgh MACS is in the tenth year of prospective follow-up of 1,199 gay/bisexual men from the Pittsburgh Tri-State area. The compelling scientific justification for the continuation of this study is that we have observed, to date, only about one-half of the estimated twenty year incubation period of HIV. Further prospective follow-up will be required in order to observe important changes in the natural history of HIV infection during the latter half of this long incubation period disease. Therefore, our broad specific aims are 1) To maintain the epidemiological framework of the Pittsburgh MACS in order to conduct further natural history and pathogenesis studies; 2) To perform the RFA-required core laboratory testing on HIV seropositive, high-risk seronegative and seronegative laboratory controls; 3) To collect and store biological specimens (blood, plasma, and cells) to further natural history and pathogenesis studies; 4) To further characterize the natural history of HIV disease, including both neurological and malignancy outcomes; 5) To continue to provide MACS-wide coordination of malignancy and autopsy studies, including the maintenance of tissue storage in Pittsburgh; 6) To analyze health services data in order to study the economic and policy implications of HIV disease. To accomplish these goals, current Pittsburgh MACS participants will be asked in October, 1994 to continue their study participation with enrollment beginning April 1, 1995. A seamless transition is expected because these dates correspond to the start and finish of the current MACS six-month visit cycle. Participants will be seen at six month intervals with epidemiologic, psychosocial, neurological, malignancy and other outcome information elicited per standard MACS protocols. HIV-infected men with fewer than 200 CD4 T-cells per mm3 will be followed at three month intervals in order to maximize capture of all HIV outcomes, including autopsy collection. Rigorous attention to study protocols and confidentiality safeguards will be maintained to extend current success in long term follow-up.